This invention is related to plasma cutting and welding torches, and more particularly to a technique for acquiring an initial torch height.
Plasma torches, also known as electric arc torches, are commonly used for cutting, welding and spray bonding of work pieces, and operate by directing a plasma consisting of ionized gas particles towards a workpiece. In the operation of a typical plasma torch, such as illustrated in U.S. Pat. No. 3,813,510, assigned to the same assignee as the present invention, a gas to be ionized is supplied to the end of the torch. A sufficiently high voltage is applied between a welding tip and an electrode at the end of the torch to cause a spark gap to jump therebetween, thus heating and ionizing the gas. A pilot, or non-transferred, arc is established by maintaining a d.c. voltage between the electrode and welding tip.
The pilot arc extends a distance from the torch tip dependent upon the power in the gap, which is in turn dependent on factors such as the pilot current, pilot voltage, torch nozzle size and, to a lesser extent, plasma gas pressure. This pilot arc provides a source of light which enables the operator to see the proper position for the torch before starting the cutting or welding operation and also provides a means for establishing a main arc. As the torch is brought towards the workpiece, the main cutting or welding arc will jump from the electrode to the workpiece since the impedance of the current path through the pilot arc to the workpiece is lower than that through air to the workpiece.
The quality of the cut in the workpiece will depend on a number of factors, including the arc current, the type of metal, the thickness of the metal, the torch height above the metal and the torch speed along the workpiece. For a given workpiece, the current, height and torch speed are set to determine the quality of the cut. Several techniques have been developed for acquiring an initial cutting height and maintaining this height during the cutting operation. None of these techniques, however, has proven entirely satisfactory for operation both in air and under water.
With mechanical sensing devices, a probe connected to a switch extends downwardly along the side of the torch body. When the torch reaches a specific height, the probe contacts the workpiece and activates the switch, interrupting the downward movement of the torch. Such devices are unsatisfactory in that the probe must be adjusted mechanically to provide the correct torch height, the probe drags along the plate during the cut and is subject to damage from the cutting debris, and the probe is offset from the center of the arc path and will not sense height correctly on a badly warped plate.
Inductive sensing devices typically comprise a large ring positioned around the torch. The inductor is part of an oscillator which changes frequency in response to changes in the inductance, and the frequency changes are used to control the torch height. Significant disadvantages of inductive sensing devices are their considerable bulk and the likelihood of damage from cutting debris.
In capacitive torch height control devices, the workpiece is used as one plate of a capacitor in an oscillator circuit. The frequency of the oscillator circuit changes with torch-to-workpiece distance, and this frequency is compared to a reference frequency to determine the proper position of the torch. These devices are disadvantageous in that electrical noise caused by the arc can interfere with the accuracy of operation and, like the mechanical and inductive sensing devices, they are offset from the cutting path and are subject to damage from cutting debris. Further, in many instances, the work piece to be cut is under water. Since the water is contaminated with cutting debris, it will be conductive to some extent and may look very much like the workpiece to the capacitive sensing system. Thus, the torch may initially set up above the surface of the water.
In a gas backpressure height control system, the gas backpressure in the torch is monitored for changes which occur as the torch approaches the workpiece. A significant disadvantage of the backpressure height control is that the gas backpressure will change not only as the torch approaches the workpiece but when it approaches other objects as well. Thus, when the work piece is under water, the gas backpressure will change as the torch approaches the surface of the water, and the downward movement of the torch may stop slightly above the water surface rather than slightly above the workpiece.
It is known that the main arc voltage existing between the torch and the work piece during the cutting or welding operation is a function of several variables, one of which is the torch height above the workpiece. Since it is possible to maintain the additional variables, e.g. a gas pressure, arc current, horizontal travel speed, type and thickness of the workpiece, etc., substantially constant during the cutting operation, height control systems have been developed which utilize the detected main arc voltage to monitor and control the torch height. Such systems, however, cannot be used to regulate height until after the cutting arc has started and stabilized and a cut has been made. This is due to transients which exist in the arc voltage until the arc stabilizes. Further, arc voltage sensing systems can only regulate torch height after they are properly set and a reference voltage is provided for comparing the sensed arc voltage. Thus, it is still necessary to acquire the initial torch height in order to determine the proper reference voltage.
A technique for acquiring initial torch height in an arc voltage torch height control system is disclosed in U.S. Pat. No. 4,170,727, assigned to the same assignee as the present invention. As disclosed therein, the power supply bridge voltage during the time of the non-transferred, or pilot, arc is used to acquire the initial height. This pilot arc voltage will increase as the torch approaches the workpiece, and the downward movement of the torch is stopped when the pilot arc voltage reaches a predetermined level. Again, however, the pilot arc voltage is dependent upon the capacitance between the torch and the workpiece and, as in the above described capacitive-type height control systems, the conductive water surface will result in faulty operation if the workpiece is under water.